Electrostrictive composite and electrostrictive element using the same

ABSTRACT

An electrostrictive composite includes a flexible polymer matrix and a carbon nanotube film structure. The carbon nanotube film structure is located on a surface of the flexible polymer matrix, and at least partly embedded into the flexible polymer matrix through the first surface. The carbon nanotube film structure includes a plurality of carbon nanotubes combined by van der Waals attractive force therebetween.

RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 200910110312.7, filed on Oct. 22, 2009 inthe China Intellectual Property Office, hereby incorporated byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to electrostrictive composites, andparticularly, to a carbon nanotube based electrostrictive composite andan actuator using the same.

2. Description of Related Art

An actuator is a device that converts input energy to mechanical outputenergy. For example, actuators can be classified into electrostatic,electromagnetic, and electrothermic type actuators.

A typical electrothermic type actuator has a double-layer structure andincludes two metallic layers having different thermal expansioncoefficients. When a current is applied, the electrothermic typeactuator bends because the thermal expansion coefficients of the twometallic layers are different. However, the electrothermic type actuatorhas a slow thermal response because flexibility of the metallic layer isrelatively poor.

What is needed, therefore, is to provide an electrostrictive compositehaving a fast thermal response, and an electrostrictive element usingthe same.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referencesto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic view of one embodiment of an electrostrictivecomposite.

FIG. 2 is a schematic, cross-sectional view, taken along a line II-II ofFIG. 1.

FIG. 3 is a schematic, cross-sectional view of another embodiment of anelectrostrictive composite.

FIG. 4 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film which can be used in the electrostrictive composite ofFIG. 1.

FIG. 5 is a schematic view of a carbon nanotube segment in the drawncarbon nanotube film of FIG. 4.

FIG. 6 is an SEM image of a flocculated carbon nanotube film.

FIG. 7 is an SEM image of a pressed carbon nanotube film.

FIG. 8 is a schematic view of the electrostrictive composite of FIG. 1before and after expansion.

FIG. 9 is a schematic structural view of one embodiment of anelectrostrictive element.

FIG. 10 is a schematic, cross-sectional view, taken along line X-X ofFIG. 9.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1 and FIG. 2, one embodiment of an electrostrictivecomposite 10 includes a flexible polymer matrix 14 and a carbon nanotubefilm structure 12. The electrostrictive composite 10 and the carbonnanotube film structure 12 are sheets. The structure 12 includes aplurality of carbon nanotubes 122 combined by van der Waals attractiveforce therebetween. The carbon nanotube film structure 12 is disposed ona first surface of the flexible polymer matrix 14. The carbon nanotubefilm structure 12 is at least partly embedded into the flexible polymermatrix 14 through the first surface of the flexible polymer matrix 14.The carbon nanotube film structure 12 has a thermal expansioncoefficient less than that of the flexible polymer matrix 14. Athickness of the electrostrictive composite 10 can range from about 20micrometers to about 5 millimeters. The thermal expansion coefficientsof the flexible polymer matrix 14 and carbon nanotube film structure 12are different.

Referring to FIG. 3, in one embodiment, the electrostrictive composite10 is a rectangular plate with the carbon nanotube film structure 12entirely embedded in the flexible polymer matrix 14. The polymer matrix14 has a dividing plane 18 which is a central plane parallel to thesurface of the polymer matrix 14. The dividing plane 18 divides thepolymer matrix 14 into two approximately symmetrical parts. The carbonnanotube film structure 12 is not disposed on the dividing plane 18 ofthe flexible polymer matrix 14. The carbon nanotube film structure 12 isoffset from a central plane of the flexible polymer matrix 14. Thus, thethermal expansion coefficients on opposite sides of the dividing plane18 of the electrostrictive composite 10 are different.

A thickness of the flexible polymer matrix 14 can range from about 20micrometers to about 4 millimeters. A material of the flexible polymermatrix 14 can be silicone elastomer, poly methyl methacrylate,polyurethane, epoxy resin, polypropylene acid ethyl ester, acrylic acidester, polystyrene, polybutadiene, polyacrylonitrile, polyaniline,polypyrrole, polythiophene or combinations thereof. In one embodiment,the flexible polymer matrix 14 is a rectangular plate made of siliconeelastomer with a thickness of about 0.7 millimeters, a length of about60 millimeters, and a width of about 30 millimeters.

The carbon nanotube film structure 12 can be a free-standing structure,that is, the carbon nanotube film structure 12 can be supported byitself and does not need a substrate for support. If a point of thecarbon nanotube film structure 12 is held, the entire carbon nanotubefilm structure 12 can be supported from that point without beingdamaged. The carbon nanotube film structure 12 includes a plurality ofcarbon nanotubes 122 combined by van der Waals attractive forcetherebetween. The carbon nanotube film structure 12 can be asubstantially pure structure consisting of the carbon nanotubes 122 withfew impurities. In one embodiment, the entire carbon nanotube filmstructure 12 is attached on the top surface of the flexible polymermatrix 14. Alternatively, the carbon nanotube film structure 12 includesa plurality of micropores and the flexible polymer matrix 14 ispermeated in the micropores of the carbon nanotube film structure 12.

In one embodiment, the carbon nanotube film structure 12 includes atleast one drawn carbon nanotube film. A film can be drawn from a carbonnanotube array, to obtain a drawn carbon nanotube film. Examples ofdrawn carbon nanotube films are taught by U.S. Pat. No. 7,045,108 toJiang et al., and WO 2007015710 to Zhang et al. The drawn carbonnanotube film includes a plurality of successive and oriented carbonnanotubes joined end-to-end by van der Waals attractive forcetherebetween. The drawn carbon nanotube film is a free-standing film.Referring to FIGS. 4 and 5, each drawn carbon nanotube film includes aplurality of successively oriented carbon nanotube segments 123 joinedend-to-end by van der Waals attractive force therebetween. Each carbonnanotube segment 123 includes a plurality of carbon nanotubes 122substantially parallel to each other, and combined by van der Waalsattractive force therebetween. As can be seen in FIG. 2, some variationscan occur in the drawn carbon nanotube film. The carbon nanotubes 122 inthe drawn carbon nanotube film are substantially oriented along apreferred orientation. The carbon nanotube film can be treated with anorganic solvent to increase the mechanical strength and toughness of thecarbon nanotube film and reduce the coefficient of friction of thecarbon nanotube film. The thickness of the carbon nanotube film canrange from about 0.5 nm to about 100 μm.

In other embodiments, the carbon nanotube film structure 12 can includetwo or more coplanar carbon nanotube films, and can include layers ofcoplanar carbon nanotube films. Additionally, when the carbon nanotubesin the carbon nanotube film are aligned along one preferred orientation(e.g., the drawn carbon nanotube film), an angle can exist between theorientations of carbon nanotubes in adjacent films, whether stacked orarrayed side by side. Adjacent carbon nanotube films can be combined byjust the van der Waals attractive force therebetween. The number oflayers of the carbon nanotube films is not limited. An angle between thealigned directions of the carbon nanotubes in two adjacent carbonnanotube films can range from about 0 degrees to about 90 degrees. Thecarbon nanotube film structure 12 in an embodiment employing these filmswill have a plurality of micropores. Stacking the carbon nanotube filmswill also add to the structural integrity of the carbon nanotube filmstructure 12.

In other embodiments, the carbon nanotube film structure 12 includes aflocculated carbon nanotube film. Referring to FIG. 6, the flocculatedcarbon nanotube film can include a plurality of long, curved, disorderedcarbon nanotubes entangled with each other. Further, the flocculatedcarbon nanotube film can be isotropic. The carbon nanotubes can besubstantially uniformly dispersed in the carbon nanotube film. Adjacentcarbon nanotubes are acted upon by van der Waals attractive force toobtain an entangled structure with micropores defined therein. It isunderstood that the flocculated carbon nanotube film is very porous.Sizes of the micropores can be less than 10 μm. The porous nature of theflocculated carbon nanotube film will increase the specific surface areaof the carbon nanotube film structure. Further, because the carbonnanotubes in the carbon film nanotube structure 12 are entangled witheach other, the carbon nanotube film structure 12 employing theflocculated carbon nanotube film has excellent durability, and can befashioned into desired shapes with a low risk to the integrity of thecarbon nanotube film structure 12. The thickness of the flocculatedcarbon nanotube film can range from about 0.5 nm to about 1 mm.

In other embodiments, the carbon nanotube film structure 12 can includea pressed carbon nanotube film. Referring to FIG. 7, the pressed carbonnanotube film can be a free-standing carbon nanotube film. The carbonnanotubes in the pressed carbon nanotube film are arranged along a samedirection or along different directions. The carbon nanotubes in thepressed carbon nanotube film can rest upon each other. Adjacent carbonnanotubes are attracted to each other and combined by van der Waalsattractive force. An angle between a primary alignment direction of thecarbon nanotubes and a surface of the pressed carbon nanotube film isabout 0 degrees to approximately 15 degrees. The greater the pressureapplied, the smaller the angle obtained. When the carbon nanotubes inthe pressed carbon nanotube film are arranged along differentdirections, the carbon nanotube film structure 12 can be isotropic.Here, “isotropic” means the carbon nanotube film has propertiessubstantially identical in all directions substantially parallel to asurface of the carbon nanotube film. The thickness of the pressed carbonnanotube film ranges from about 0.5 nm to about 1 mm. An example ofpressed carbon nanotube film is taught by US PGPub. 20080299031A1 to Liuet al.

In one embodiment of FIG. 1, the carbon nanotube film structure 12includes a plurality of drawn carbon nanotube films stacked together,the carbon nanotubes of each drawn carbon nanotube film aresubstantially aligned along one preferred orientation.

Referring to FIG. 8, the operating principle of the electrothermiccomposite 10 is described as follows. When a voltage is applied to thecarbon nanotube film structure 12 via two electrodes 16, a current flowsthrough the carbon nanotube film structure 12. The carbon nanotubes 122convert the electric energy to heat thereby heating the flexible polymermatrix 14, which allows the flexible polymer matrix 14 to expand alongfrom one electrode 16 to the other. The thermal expansion coefficientsof the flexible polymer matrix 14 and the carbon nanotube film structure12 are different so that the electrothermic composite 10 bends in adirection oriented to the carbon nanotube film structure 12 with asmaller thermal expansion coefficient.

The expansion coefficient of the electrostrictive composite 10 with anoriginal length L1 of about 34 millimeters, an original thickness ofabout 0.7 millimeters, and an original width of about 5 millimeters istested. The carbon nanotube film structure 12 has a thickness of about20 micrometers. After a voltage of 40 V is applied to theelectrostrictive composite 10 for about 2 minutes, a displacement ΔS, asshown in FIG. 8, is about 16 millimeters.

Referring to FIG. 9 and FIG. 10, an electrothermic element 20 accordingto one embodiment is shown. The electrothermic element 20 includes anelectrostrictive composite 10, a first electrode 22, and a secondelectrode 24. The first electrode 22 and the second electrode 24 areseparately disposed on and electrically connected to the carbon nanotubefilm structure 12.

In one embodiment, the first electrode 22 and the second electrode 24are separately located on and electrically connected to two ends of thecarbon nanotube film structure 12. The first electrode 22 and the secondelectrode 24 can be made of metal, alloy, conductive resin, indium-tinoxide (ITO), conductive adhesive, carbon nanotube, carbon grease, andany other suitable materials. The shapes of the first electrode 22 andthe second electrode 24 are arbitrary. In one embodiment, the firstelectrode 22 and the second electrode 24 are two copper sheets. In otherembodiments, the first electrode 22 and the second electrode 24 are twocopper wires.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the disclosure. Variations maybe made to the embodiments without departing from the spirit of thedisclosure as claimed. The above-described embodiments illustrate thescope of the disclosure but do not restrict the scope of the disclosure.

What is claimed is:
 1. An electrothermic element comprising: a flexiblepolymer matrix; a carbon nanotube film structure entirely embeddedwithin the flexible polymer matrix, the carbon nanotube film structurebeing located at an offset from a central dividing plane of the flexiblepolymer matrix; a first electrode; and a second electrode, wherein thefirst electrode and the second electrode are separately disposed in theflexible polymer matrix and electrically connected to the carbonnanotube film structure.
 2. The electrothermic element of claim 1,wherein the first electrode and the second electrode are electricallyconnected to two ends of the carbon nanotube film structure.
 3. Theelectrothermic element of claim 2, wherein the carbon nanotube filmstructure comprises a plurality of micropores, and the flexible polymermatrix is permeated in the plurality of micropores of the carbonnanotube film structure.
 4. The electrothermic element of claim 1,wherein the carbon nanotube film structure comprises at least one drawncarbon nanotube film comprising a plurality of carbon nanotubessubstantially parallel to each other and combined by van der Waalsattractive force therebetween.